The present invention relates to a method of measuring the surface of a workpiece using a motorised scanning head mounted on a coordinate positioning apparatus such as a coordinate measuring machine (CMM), machine tool, manual coordinate measuring arm and inspection robot.
It is known from International Patent Application No. WO90/07097 to mount a motorised scanning head on a coordinate positioning machine. The motorised scanning head enables a stylus mounted on the motorised scanning head to be rotated about two orthogonal axes. The stylus may be positioned angularly about these two axes whilst the motorised scanning head can be positioned by the coordinate positioning machine in any position within the working volume of the machine.
Such a motorised scanning head provides a coordinate positioning machine with greater scanning flexibility because the motorised scanning head can position the stylus in many different orientations.
This application discloses measurement sequences in which simple movement of the coordinate positioning apparatus is combined with movement of the motorised scanning head to measure regularly shaped parts. For example a bore is measured by moving the quill of the CMM along a centre line while the motorised scanning head moves the stylus tip in a circular profile thus resulting in a helical motion. Likewise, a plane surface may be measured by moving the quill of the CMM at constant speed parallel to the surface as the motorised scanning head performs a sweeping motion.
This also discloses scanning the surface of a cone, placed with its axis parallel to the Z axis of a CMM. The quill is driven in a circular path while the motors of the scanning head cause the surface sensing device to be biased against the cone about the Z axis.
In traditional methods of measuring workpieces, the stylus tip position is at a fixed offset from the quill and is known from probe calibration.
With a 5 axis system, the stylus tip position is dependent on the angles of the scanning head (i.e. about axes A1 and A2) and probe length, so the stylus tip position is constantly varying with respect to the quill position as the angles of the scanning head vary.
In this case the actual tip position is affected by the tolerances in the scanning head. Thus either the quill must be fixed and the stylus tip position allowed to vary from its nominal position to allow for these tolerances or visa versa.